30 Respiratory and Circulatory Systems
30 Respiratory and Circulatory Systems
30 Respiratory and Circulatory Systems
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<strong>30</strong>.4<br />
Blood Vessels <strong>and</strong> Transport<br />
KEY CONCEPT The circulatory system transports materials throughout the body.<br />
MAIN IDEAS<br />
• Arteries, veins, <strong>and</strong> capillaries transport<br />
blood to all parts of the body.<br />
• Lifestyle plays a key role in circulatory<br />
diseases.<br />
VOCABULARY<br />
blood pressure, p. 923<br />
systolic pressure, p. 923<br />
diastolic pressure, p. 923<br />
Review<br />
artery, vein, capillary,<br />
ventricle<br />
Connect In the 1600s, most scientists thought that the lungs, not the heart,<br />
moved the blood, <strong>and</strong> that blood was consumed <strong>and</strong> produced by the internal<br />
organs. William Harvey, court physician to the king of Engl<strong>and</strong>, challenged these<br />
ideas. He showed that the heart was the true pump for the blood <strong>and</strong> that blood<br />
circulated in two pathways: one between the heart <strong>and</strong> the lungs, <strong>and</strong> another<br />
between the heart <strong>and</strong> the rest of the body. Harvey’s work on circulation is<br />
regarded as one of the greatest advances in the history of medicine.<br />
MAIN IDEA<br />
Arteries, veins, <strong>and</strong> capillaries transport blood to<br />
all parts of the body.<br />
As you read in Section <strong>30</strong>.1, the circulatory system includes three types of<br />
blood vessels—arteries, veins, <strong>and</strong> capillaries—that act as transportation<br />
networks for the blood. Each of the three vessels has its own structure <strong>and</strong><br />
function, as illustrated in FIGURE <strong>30</strong>.11.<br />
TAKING NOTES<br />
A two-column chart can help<br />
you organize your notes about<br />
different blood vessels <strong>and</strong> circulatory<br />
pathways.<br />
arteries<br />
- Thicker, more<br />
muscular than<br />
veins<br />
- Blood under<br />
greater<br />
pressure<br />
Arteries<br />
Arteries need to be strong <strong>and</strong> flexible because the blood they carry from the<br />
heart is under great pressure. An artery’s thick wall is composed of three<br />
layers. The innermost layer consists of endothelium coated with a protein that<br />
prevents blood from clotting. The middle layer is a thick b<strong>and</strong> of smooth<br />
muscle <strong>and</strong> elastic fibers. The outer layer consists of connective tissue <strong>and</strong><br />
elastic fibers. The elastic fibers allow the arterial walls to exp<strong>and</strong> <strong>and</strong> contract<br />
to help move blood through the arteries. Arterioles, or smaller arteries, contain<br />
the same three layers, but the outer <strong>and</strong> middle layers are much thinner.<br />
Veins<br />
The structures of veins reflect the fact that blood is under much less pressure<br />
when it is returning to the heart. Veins have larger diameters <strong>and</strong> thinner walls<br />
than do arteries <strong>and</strong> contain valves that prevent blood from flowing backwards.<br />
Veins do not have a thick layer that exp<strong>and</strong>s <strong>and</strong> contracts to keep blood<br />
moving. Instead, they need the activity of skeletal muscles to help maintain<br />
circulation. For example, as you walk, skeletal muscles in your legs push against<br />
the veins. The valves open, <strong>and</strong> blood moves toward the heart. If you sit for too<br />
long, the lack of exercise makes it harder for the blood to move upward.<br />
Venules are small veins that join larger veins to capillaries.<br />
922 Unit 9: Human Biology
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